Photovoltaic device

ABSTRACT

A photovoltaic means, e.g. a solar cell in which mobile charge carriers are generable by incident optical radiation contains a semiconductor body of a material having such an energy band structure that in the spectral range of the incident radiation an additional carrier generation is made possible by impact ionization (Auger effect). As a result, the internal quantum efficiency may be increased above the value 1. The semiconductor material may consist of germanium and silicon, particularly GeSi.

FIELD OF THE INVENTION

This application relates to photovoltaic devices specifically adaptedfor conversion of visible radiation energy, for example for solar cells.

BACKGROUND

The efficiency of the photovoltaic conversion of energy is of decisivesignificance for the practical application of photovoltaic means. Insuch classical photovoltaic means as solar cells the generation quantumefficiency (GQE), i.e. the number of electron/hole pairs generated perquantum of radiation, maximally equals 1, thus resulting in themeasurable internal quantum efficiency (IQE) also being maximally equalto 1. It has hitherto been assumed that the value 1 cannot be exceededand that the proportion of photon energy depending on the band gap ofthe semiconductor and exceeding the energy necessary for generating anelectron hole pair is lost as heat.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a device which uses theproportion of radiation quantum energy exceeding the band gap energy ina photovoltaic semiconductor means, particularly to provide improved andefficient solar cells.

Briefly, according to the invention, the photovoltaic device contains asemiconductor material having such a band structure that an additionalinternal generation of carrier pairs is able to take place by opticalimpact ionization, that is, by Auger generation or carrier generation bythe Auger effect.

A preferred semiconductor material is germanium and silicon, e.g. GeSi.

As a result of additional charge carriers being generated by impactionization, the internal quantum efficiency of the carrier generationcan be made to exceed the value 1.

For photon energies exceeding twice the fundamental energy gap E_(G) ofthe semiconductor of the photovoltaic means, an internal quantumefficiency IQE=2 and more is possible by impact ionization (Augergeneration).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail, whereby furtherfeatures and advantages of the invention will become evident, withreference to the drawings, in which:

FIG. 1 is a schematic sectional view of a photovoltaic means in the formof a solar cell in which the invention may find application;

FIG. 2 plots the ultimate (theoretically maximum) efficiency n_(ult) asa percentage dependent on the fundamental gap energy E_(G) in eVassuming an illumination by radiation having the spectrum of a blackradiator;

FIG. 3 plots the ultimate efficiency n_(ult) for the AM0 spectrum(smooth curves) and the AM1.5G spectrum (curves having several relativemaxima);

FIG. 4 is an example of a band structure E(k) with which a quantumefficiency of more than 1 may be achieved by Auger generation withradiation quantum starting energy of 2 E_(G) ; and

FIG. 5 graphically represents the band structure of the zinc blende typesemiconductor material GeSi.

DETAILED DESCRIPTION.

FIG. 1 shows, greatly simplified, the structure of a typical solar cellin section. The solar cell 10 represented as a typical example includesa wafer-like monocrystalline semiconductor body 12 of silicon orpreferably of GeSi comprising an e.g. 280 μm thick 100!-oriented,p-conducting main part 14 and an e.g. 0.4 μm thick phosphorous-dopedzone 16. The main part 14 is arranged on a substrate electrode 18, andon the surface zone 16 a transparent or structured electrode (not shown)and an e.g. 100 nm thick, thermally generated protective layer of SiO₂20 are provided.

FIG. 2 shows the maximum possible efficiency of solar cell 10 as afunction of band gap energy for the illumination spectrum of a blackbody having a temperature of 6000 K for a maximum internal quantumefficiency IQE_(max) =1 for hν≧E_(G), furthermore for IQE_(max) =1.5 forhν≧2E_(G) and for IQE_(max) =2 for hν≧2E_(G). For the last case, theprimary band gap as an optimum for the photovoltaic energy conversionshifts from approx. 1 eV to values below 1 eV whilst at the same timethe maximum possible efficiency of 44.7% for the case IQE_(max) =1increases to 60% for IQE_(max) =2. Semiconductor materials in whicheffective additional charge carriers are generated by the Auger effect(impact ionization), by making better use of the energies of irradiatedphotons, may thus result in considerable increases in the efficiency ofphotocell.

FIG. 3 shows the maximum possible efficiency as a function of band gapenergy for the AM0 spectrum (total intensity 1350 W/m²) and the AM1.5spectrum (total intensity 1000 W/m²) assuming IQE=1 (no Augergeneration) and IQE_(max) =1.5 as well as IQE_(max) =2 assuming Augergeneration.

The maximum value of the ultimate efficiency for the AM0 spectrum shiftsto smaller energies of the energy gap when taking into account the Augergeneration, the optimum band gap for this spectrum amounting to 1.35 eVfor IQE_(max) =1. With Auger generation (assuming IQE_(max) =1.5) theoptimum band gap shifts to 1.2 eV or for even more effective Augergeneration (assuming IQE_(max) =2) to even 0.9 eV. Also, the tworelative maxima of the ultimate efficiency for the AM1.5 spectrum shiftin a similar way, their still being of the order 1.4 eV and 1.1 eVassuming IQE_(max) =1, but shift to 0.9 eV and 1.1 eV (assumingIQE_(max) =1.5) or to 0.8 eV and 0.9 eV for IQE_(max) =2 when takingAuger generation into account. At the same time, the relative maxima arereduced in the curves relating to the AM1.5 spectrum, the creation ofwhich is caused by the sharp breaks in the IR range of the AM1.5Gspectrum due to H₂ O and CO₂ absorption lines. This behavior is alsoattributable to the better use made of the energy of each photon byAuger generation.

For a radiation quantum energy equivalent to twice the band gap (IQE=2)the ultimate efficiency with Auger generation is increased by a factorof almost 1.5. A material in which an Auger generation, i.e. impactionization is possible, is thus superior by a factor of almost 1.5 to amaterial having the same energy gap but having no possibility of Augergeneration.

In practice, the aforementioned ultimate efficiencies are reduced byrecombination processes such as e.g. radiative recombination and Augerrecombination. For the indirect semiconductor silicon it has been foundthat it is not the radiative recombination but the Auger recombinationwhich is the process determining the limit of the no-load voltage of thephotovoltaic means.

By exploiting Auger generation, i.e. the additional generation ofcarrier pairs, the theoretical maximum achievable efficiency for thespectrum of the black body may be enhanced from 44% to 60%. At the sametime, the maxima of the ultimate efficiency, i.e. the optimum band gapfor a photovoltaic means designed for the spectrum of the black body,are shifted to lower energies for the AM0 spectrum and the AM1.5spectrum. It is thus not values of 1.1 eV (Si) and 1.4 eV (GaAs) whichprovide the best conditions for the photoconversion with radiation inthe AM1.5 spectrum but semiconductor materials having smaller energyband gaps when exploiting Auger generation. The shift in the optimumband gap towards lower energies becomes all the more pronounced, themore effective the Auger generation and, accordingly, also theexploitation of the energy of each photon is.

The conditions for an optimum band structure of a semiconductor materialfor photovoltaic means such as solar cells, taking Auger generation intoaccount (impact ionization, internal photoeffect) making it possible toachieve high quantum efficiencies of IQE>1 over a broad range of thesolar spectrum will now be explained in the following.

The conditions for an energetic wideband-effective impact ionization inthe semiconductor material, of which as many as possible are to beachieved, are:

a) The semiconductor should have both a lowest indirect band gap andalso as many direct band gaps as possible, which should be greater thanthe indirect band gap.

b) Employing impact ionization (Auger generation) for generating pairsof charge carriers should be done at low energies where possible.

c) The excess energy E_(x) =hν-E_(G) should ideally always be to thebenfit of only a single charge carrier of the pair of charge carriersgenerated primarily (principle of unequal distribution of excessenergies).

d) Points in the band structure offering the primary charge carrier,i.e. electron or hole favorable possibilities of triggering secondarypairs of charge carriers by Auger generation should be promoted bystrong absorption of light (zones L, Γ, X in FIG. 4). As compared tothis, points in the band structure having unfavorable conditions forAuger generation (zone F in FIG. 4) should be activated only weakly orshould be energetically degenerated with points of a high impactprobability (zone L in FIG. 4).

e) Optical transitions leading to favorable starting positions for Augergeneration should adjoin each other energetically as of the startingenergy and cover the solar spectrum as wideband as possible.

These criteria can be rendered more concrete by the requirements on theband structure as given below. In this respect the condition a) resultsin restriction to the highest valence band E_(v) and the lowestconduction band E_(c) of an indirect semiconductor. Parameters importantto this consideration are the energetic differences of a charge carrierin the E(k) profile to the band extremum in each case, wherein thek-dependent valence band difference ΔE_(v) (k) being the differencebetween the valence band maximum and the valence band for each wavevector k in accordance with ΔE_(v) (k)=E_(v).max -E_(v) (k). Thek-dependent conduction band difference E_(c) (k) is the differencebetween conduction band and conduction band minimum E_(c).min for eachwave vector k in accordance with ΔE_(c) (k)=E_(c) (k)-E_(c).min.

Generating two electron/hole pairs necessitates a photon energy of atleast twice the energy of the fundamental (indirect) band gap andrepresents the minimum starting energy for Auger generation. If a directtransition exists in this photon energy then a strong optical transitionis present.

Requirement 1: (Starting energy of Auger generation) The first directband gap E_(D1) amounts to at least twice the fundamental (indirect)band gap E_(G) according to

    E.sub.D1 =2E.sub.G.                                        (1)

Auger generation commences when E_(D1) in addition to either theconduction band difference or the valence band difference becomes zeroin accordance with

    ΔE.sub.v =0 or ΔE.sub.c =0,                    (2)

so that either the electron or the hole alone takes over the excessenergy in its entirety. In this way no portion of the photon energy islost, the energy in its entirety being used in generating electron/holepairs.

At as many loci of the Brillouin zone as possible high values of ΔE_(c)should correspond with low values of ΔE_(v) and vice-versa. The excessenergy E_(x) =hν-E_(G) should always be released where possible totallyto either the electron or the hole of the resulting charge carrier pairto satisfy the minimum energy requirement of Auger generation at manypoints.

Requirement 2: (Minimum energies of Auger generation) The k-dependentenergy differences in the conduction band attain maximum values when theenergy differences in the valence band are a minimum or equal zero andvice-versa. These are just the properties of an indirect semiconductor.Minimum values of band differences leading to impact ionization are

    ΔE.sub.c >E.sub.G, when ΔE.sub.v =0, and       (3)

    ΔE.sub.v >E.sub.G, when ΔE.sub.c =0            (4)

applies.

At loci at which neither the conduction band nor the valence banddifference ΔE_(c), ΔE_(v) equals zero, the minimum band gap in this caseamounts to

    ΔE(k)=E.sub.c (k) -E.sub.v (k)=2E.sub.G +2ΔE.sub.v for ΔE.sub.v ≠0 and                               (5)

    ΔE(k)=E.sub.c (k)-E.sub.v (k)=2E.sub.G +2ΔE.sub.C ≠0 (6)

Requirement 3: (Effective points of Auger generation)

Points at which one of the minimum band gaps of the equations (2)-(6) isexceeded should be excited by strong optical transitions. This is thecase when these are located at points of high symmetry or the localcurvature of valence band dE_(v) /d|k| and conduction band dE_(c) /d|k|is the same.

For points having ##EQU1## should apply.

Loci which due to an equipartition of the excess energy ΔE_(c)(k)≈ΔE_(v) (k) are unfavorable for Auger generation and all loci notsatisfying the minimum requirements on the band differences stemmingfrom requirement 2 should not be located at points of high symmetry butalong the axes. The curvature of conduction band and valence band shouldbe different where possible so that here the absorption is only weak.

For points having ##EQU2## should apply.

If, in addition to this, the band curvature at the band extremum isslight, according to ##EQU3## there are numerous possibilities for thehot charge carriers in the band extremum to relax by the Auger effectand to generate additional charge carriers. Strong optical transitionsfor simultaneously strong (but equal) dispersion of both bands alsoresult in high impact ionization probabilities if the dispersion in theband extremum is slight.

Requirement 4: (Distribution of effective points of Auger generationover the solar spectrum)

Direct transitions having large band differences which satisfy theminimum energies of requirement 2 and which are occupied by strongoptical transitions in accordance with equation (9) adjoin each otherenergetically and cover the solar spectrum over a wide band, inaccordance with

    2E.sub.G =E.sub.D1 <E.sub.D2 <E.sub.D3 <E.sub.D4. . . ≦hν.sub.max.                                    (14)

The maximum photon energy hν_(max) amounts to 4 eV for the AM1.5Gspectrum and about 10 eV for the AM0 spectrum.

FIG. 4 is an example of a semiconductor band structure with which thequantum efficiency above the value 1 may be achieved by Auger generationfor a starting energy of 2E_(G). For a photon energy of 2E_(G) there arenumerous direct transitions which result in favorable starting positionsfor Auger generation from either the primary generated electron or hole.

FIG. 4 shows the band structure of a hypothetical semiconductor whichsatisfies the above principles ideally: an indirect semiconductor havingthe fundamental energy gap E_(G) (from Γ to X) and having the firstdirect transition at 2E_(G) (at the Γ point). There are adjoining directtransitions of 2.5E_(G) (at L and 3.5E_(G) (at X) as well as 3E_(G)(along the Σ axis). The transitions at X and L lead to hot holes, thoseat Γ and along the Σ axis lead to hot electrons. The charge carriershave sufficient excess energy at these points to relax to the bandextrema. The point F along the Δ axis having an unfavorable distributionof excess energies (neither E_(c) nor ΔE_(v) exceeds the indirect bandgap E_(G)) is energetically degenerated with the transition at the Lpoint which results in a highly favorable distribution of the excessenergies to electron and hole.

With the aim that the gain in current being possible by Auger generationcannot be compensated by a loss in the voltage as a result of an alsopromoted Auger recombination, the end points of the Auger generation(these being of course at the same time the starting points of Augerrecombination) should not be located at the absolute extreme of the bandbut instead should be somewhat removed therefrom. This results in thestarting points of the recombination under V_(oc) conditions not beingoccupied and thus no Auger recombination can take place.

To be able to make use of the advantages of impact ionization, otherpossibilities of the energy loss of hot charge carriers must be renderedineffective where possible. The interaction with phonons should be asweak as possible, i.e. the bond relationships should be unpolar wherepossible. The energy addressed to other charge carriers by theinteraction between charge carrier/charge carrier interaction may beinfluenced by the doping.

The optimum band structure is characterized by strong opticaltransitions over a broad range. They result in a high extinctioncoefficient k and thus in small optical penetration depths. Smalloptical penetration depths necessitate good possibilities of surfacepassivation so that the charge carriers generated in the vicinity of thesurface do not instantly recombine. This strong absorption also resultsin a high reflection of the semiconductor. To achieve effective use bythe additionally generated charge carriers the reflection would have tobe reduced by suitable antireflection coatings or, otherwise, byproviding multiple absorption chances for reflected light rays as aresult of non-planar surfaces (light traps).

Accordingly, the properties of the band structure of a semiconductorcapable of permitting Auger generation and thus quantum efficienciesgreater than the value 1 at low recombination are:

pronounced parallelism of the uppermost conduction band and thelowermost valence band;

generally, a strong dispersion with conduction band and valence banddifferences which exceed the fundamental band gap;

locally weak and similar dispersion in valence band and conduction bandin the vicinity of the points of symmetry and in the vicinity of theband extrema;

the starting energy of Auger generation at an optimum, because of itbeing as low as possible, is E_(D1) =2E_(G) (for ΔE_(v) =0 or ΔE_(c)=0);

points having good possibilities of Auger generation adjoin each otherenergetically.

These requirements as to the band structure are either in keeping withother requirements on solar cells (e.g. high minority carrier lifetimes)or, if in contradiction with the latter, can be recompensated by methodsalready available (e.g. antireflection coatings).

A semiconductor well satisfying the requirements as given above is thezinc blende-type semiconductor GeSi. FIG. 5 shows the band structure ofthis semiconductor calculated by Schmid et al. (U. Schmid, N. E.Christensen and M. Cardona, Phys. Rev. Vol. 41, 5919 (1990)). Favorablepoints for Auger generation are indicated by arrows. GeSi is an indirectsemiconductor having a energy gap of 1.2 eV between X₆ and Γ₈. Thepronounced parallelism of the lowermost conduction band and theuppermost valence band is remarkable, which feature is similar to thecase of silicon and being favorable for impact ionization. One majoradvantage afforded by GeSi as compared to Si is the energy of the firstdirect transition at the Γ point which at 2.4 eV as compared to 3.4 eVfor Si relates substantially more favorably to the solar spectrum andsatisfies especially the minimum energy condition. Starting of Augergeneration is thus shifted to the lowest possible energy for a givenfundamental energy gap. The end point of impact ionization starting fromthe Γ point is the X point. In its vicinity the dispersion of theconduction band in the Δ direction is merely weak, this making numerousimpact ionization transitions possible which should result in a highimpact ionization probability.

The valence band difference at L also exceeds E_(G), however,requirement 5), which in this case is more stringent for thenon-disappearing conduction band difference, cannot be satisfied, impactionization within the first conduction and valence band thus not beingpossible starting from L. Along the Σ axis there is also a region at theΓ point which satisfies the energy condition ΔE_(c) >E_(G). Only partsof these points (in the vicinity of Γ) result in numerous impactionization transitions. The reason for this is the stronger dispersionat the end point of impact ionization (i.e. at the X point in the Σdirection) which results in a reduction of the possibilities for impactionization. In the vicinity of the X point an extensive region satisfiesthe requirement 4). The low dispersion of the valence band at the Γpoint in both the Δ and Σ direction permits numerous impact ionizationtransitions and should result in a high impact ionization probabilityfor holes excited at these points. Direct transitions exist at Γ andalong the Σ axis in the vicinity of Γ which result in high impactionization probabilities of hot electrons, as well as direct transitionsexist at X and near thereto along the Δ and Σ axis which result in highimpact ionization probabilities of hot holes. These direct transitionsadjoin each other energetically and thus cause good impact ionizationefficiencies in a broad energy interval as of the starting energy of thefirst direct band gap of 2.4 eV.

As a zinc blende semiconductor, GeSi has less symmetry than silicon,which in general results in less degeneration of the energy bands. Ifthis greater degeneration of the energy bands causes the direct opticaltransitions to be somewhat weakened as compared to those in silicon,this would make demands on the surface passivation which are not so highas is the case of Si. Nevertheless, the hitherto difficult passivationof the GeSi surface for making use of Auger generation in a possibleGeSi solar cell remains the prime technological task needing to besatisfied.

In an experiment with 0.92 eV (R. Braunstein, A. Moore and F. Herman,Phys. Rev. 109, 685 (1958)) the value of the energy gap turns out to beless than the value calculated by Schmid. The value determinedexperimentally is in good agreement with the optimum band gap assumingAuger generation.

This band gap can be further optimized by varying the Ge/Si ratio.

As a photoconductive layer a stressed SiGe layer may also be used,wherein compressive and tensile stresses can be incorporated in thelayer. Incorporating stresses in this way can be done by epitaxying theSiGe layer on substrates having smaller lattice constants (e.g. Si) orgreater lattice constants (e.g. Ge) to thus reduce or increaserespectively the lattice constant of the SiGe layer with respect to thenon-stressed layer.

The invention is applicable to the many and varied photovoltaic meanse.g. barrier-layer photocells, Schottky contact photocells, meansincorporating heterostructured semiconductor bodies, and the like.

We claim:
 1. A photovoltaic device with a semiconductor body in whichmobile charge carriers are generatable by incident opticalradiation,said semiconductor body containing a monocrystalline materialwith an energy band having both a fundamental indirect band gap E_(G)and at least one larger direct band gap being located at a specificpoint in the Brillouin zone of the material, said Brillouin zone beingthe unit cell of the reciprocal lattice of the crystalline material,wherein at least one direct band gap has an energetic differencecorresponding to the visible range of electromagnetic radiation and istwice as large as said indirect band gap, and for said specific point inthe Brillouin zone either relationship

    ΔE.sub.v =0 or ΔE.sub.c =0,

is valid, wherein ΔE_(v) is the energetic difference between the valenceband at said specific point and the valence band maximum, and ΔE_(c) isthe energetic difference between the conduction band at said specificpoint and the conduction band minium.
 2. The device as set forth inclaim 1, characterized in that at at least one further locus in theBrillouin zone of a material a larger value of ΔE_(c) corresponds to asmaller value of ΔE_(v) and that

    ΔE.sub.c ≧E.sub.G, when ΔE.sub.v =0 and

    ΔE.sub.v ≧E.sub.G, when ΔE.sub.c =0.


3. The device as set forth in claim 2, characterized in that at loci inthe Brillouin zone of the material at which neither ΔE_(c) nor ΔE_(v)equals zero the minimum band gap ΔE(k) amounts to:

    ΔE(k)=E.sub.c (k)-E.sub.v (k)=2E.sub.G +2ΔE .sub.v for ΔE.sub.v ≠0 and

    ΔE(k)=E.sub.c (k)-E.sub.v (k)=2E.sub.G +2ΔE .sub.c for ΔE.sub.c ≠0 .


4. The device as set forth in claim 2, characterized in that said lociin the Brillouin zone of the material at which

    ΔE.sub.v ≈ΔE.sub.c <E.sub.G

applies, are not located at points of high symmetry, or that at theseloci dE_(v) /d|k| is substantially unequal to dE_(c) /d|k|.
 5. Thedevice as set forth in claim 1, characterized in that said direct bandgaps and/or the loci satisfying the further conditions are located atpoints of high symmetry or in that at these dE_(v) /d|k| substantiallyequals dE_(c) /d|k|.
 6. The device as set forth in claim 1,characterized in that said semiconductor material comprises severaldirect band gaps substantially adjoining to each other.
 7. The device asset forth in claim 1, wherein said semiconductor material comprisesgermanium and silicon.
 8. The device as set forth in claim 1,characterized in that said semiconductor material consists of germaniumand silicon.
 9. The device as set forth in claim 1, characterized inthat said semiconductor material consists of SiGe.